"Are they additional boys?"
Translation:To jsou další kluci?
In such sentence, "to" is used for anything - boys, girls, one boy, one girl, refrigerator, freedom, anything... ;) "Ti (kluci)" is a demonstrative plurar masculine pronoun used only in front of a noun.
"To jsou"/"jsou to" is a special construction you will meet in hundreds of sentences in this course and it was explained in many many many other sentences. Please search the forum.
I will just stress it is always TO. To jsem já, To jsi ty, To je on. To jsme my. To jste vy. To jsou oni. To jsou velcí lidé. Always the same TO.
It is translated as This is... These are... Those are...